“More is different,” observed physicist Philip Anderson. He meant that the collective behavior of quantum many-body systems can produce entirely new physical effects, for example, the emergence of low-energy excitations carrying a fraction of the quantum of electric charge—the charge of an electron. To see this behavior happen at the most fundamental level of individual particles, scientists have built quantum simulators—machines that control individual atoms to mimic the properties of many-body ensembles of electrons. Now Philipp Lunt of Heidelberg University in Germany and his collaborators have gotten considerably closer to seeing how fractional charges emerge [1]. They emptied a metaphorical cocktail glass (an optical tweezer filled with ultracold fermions) to leave just one sip (a single pair of atoms). They stirred the remnants, mimicking the effect that a magnetic field has on real electrons, and thereby created something even more exciting: a cocktail of a strongly correlated atomic pair in a state whose wave function matches the one that physicist Robert Laughlin devised to describe the fractional quantum Hall effect. Besides being collective, the Laughlin state is also topological. The feat portends the use of cold atoms to dissect other, more exotic topological states, such as quantum Hall ferromagnets or topological p-wave superconductors.
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